1. Field of the Invention
The present invention relates to a fuel cell. More particularly, the present invention relates to a PEFC (Polymer Electrolyte Fuel Cell) which can humidify an electrolyte membrane of the cell by itself.
2. Description of Related Art
A PEFC apparatus includes individual fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes include an anode provided on one side of the membrane and constructed of a first catalyst layer and a first diffusion layer, and a cathode provided on the other side of the membrane and constructed of a second catalyst layer and a second diffusion layer. The separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and for supplying oxidant gas (oxygen, usually, air) to the cathode. A plurality of fuel cells are piled to construct a module. A number of modules are piled, and electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules to construct a stack of fuel cells. After tightening the stack of fuel cells between the opposite end plates in a fuel cell stacking direction, the end plates are coupled to the fastening member (for example, a tension plate) extending in a fuel cell stacking direction outside the pile of fuel cells by bolts extending perpendicularly to the fuel cell stacking direction.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator) to form water as follows:
At the anode: H2xe2x86x922H++2exe2x88x92
the cathode: 2H++2exe2x88x92+(xc2xd)O2xe2x86x92H2O
In order that the hydrogen ions move through the electrolyte, the electrolyte membrane has to be aqueous. If the aqueous concentration decreases, the electric resistance of the electrolyte membrane increases accompanied by a decrease in an output voltage and a decrease in an output power.
To cool the fuel cells, the temperature of which rises due to the heat generated at the water production reaction and a Joulean heat, a cooling water passage is formed at every cell or at every module and a cooling water is caused to flow in the cooling water passage. Along the flow direction of the cooling water, a low-temperature portion (at about 75xc2x0 C.) at the upstream portion and a high-temperature portion (at about 85xc2x0 C.) at the downstream portion are caused in the cell.
To maintain the aqueous condition of the electrolyte membrane, usually, the hydrogen and the air supplied to the cell are humidified by respective humidifiers before they are supplied to the cells. Japanese Patent Publication No. HEI 7-320755 discloses that the fuel gas is humidified and that the fuel gas is caused to flow in the cell from a high-temperature portion to a low-temperature portion so that a distribution of the relative humidity of the fuel gas along the fuel gas passage is uniform whereby the power output of the fuel cell is improved.
However, since no product water is produced at the anode unlike at the cathode and therefore not so much water exists in the fuel gas as in the oxidant gas, even if the humidity distribution control of the fuel gas is conducted like Japanese Patent Publication No. HEI 7-320755, there is little effect in obtaining a uniformly aqueous condition of the electrolyte membrane over the entire area of the cell, and so it is yet necessary to provide humidifiers for the fuel gas and the oxidant gas.
An object of the present invention is to provide a fuel cell capable of conducting a self-humidification of the cell utilizing the product water of the cell and operable stably with no separate humidifier.
A fuel cell according to the present invention is of a PEFC-type and includes a cell plane with a high-temperature portion and a low-temperature portion. The fuel cell includes an oxidant gas passage where an oxidant gas flows. The oxidant gas passage is formed in the cell and extends parallel to the cell plane. An oxidant gas flow direction is directed from the high-temperature portion to the low-temperature portion so that a product water recirculates in the oxidant gas passage.
The oxidant gas flow direction may be reverse to the direction of gravity.
The oxidant gas passage includes an upstream portion and a downstream portion. The upstream portion of the oxidant gas passage may be provided with a hydrophilicity, and the downstream portion of the oxidant gas passage may be provided with a hydrophobicity.
The fuel cell further includes a fuel gas passage where a fuel gas of pure hydrogen flows. The fuel gas passage is formed in the cell and extends parallel to the cell plane. A fuel gas flow direction may be directed from the high-temperature portion to the low-temperature portion.
With the above fuel cell according to the present invention, since the oxidant gas flow direction is directed from a high-temperature portion to a low-temperature portion of the cell, a water vapor of the product water is condensed to a water drop at a gas outlet located at the low-temperature portion. Then, the water drop recirculates to a gas inlet located at the high-temperature portion where the water drop is evaporated and raises the humidity of the oxidant gas. By this mechanism, the product water recirculates in the cell plane and conducts a self-humidification of the cell, whereby the fuel cell is stably operated even with no humidifier.